I'm not sure how adding an email address field changes the fact that you allow users to enter a code to unlock content or functionality?
Are there any apps you know of that do what you want to do? If not and if Apple says you can't do it, you will most likely need to change your requirements for this. If using App Store Connect code functionality doesn't work, you probably need to move the code redemption to your web site (like Marvel at https://marvel.com/redeem/ ).
Topic:
App & System Services
SubTopic:
StoreKit
Tags: